Kinetic study of the gas-phase decomposition of the trifluoroacetyl radical. Effects of temperature and pressure upon the rate constants

Abstract

The kinetics and pressure dependence of the decomposition of the trifluoroacetyl radical: CF₃CO + M→CF₃+CO + M (5′), have been studied by generating the radicals from the selective photolysis of azomethane in the presence of 1,1,1-trifluoroacetaldehyde over the temperature range 338–417 K. The rate constants of the decomposition reaction (k_5′) have been measured relative to the radical combination reactions: CH₃+CF₃CO→CF₃COCH₃(6), 2CH₃→C₂H₆. (2)

The rate constants (k_5′), were found to be pressure-dependent over the pressure range 50–740 Torr, with SF₆ as a bath gas.

The A factor of the radical decomposition reaction has been calculated from transition-state theory to be A^∞_5′≈ 2.2 × 10¹³ s^–1. Treatment of the pressure dependence of the rate constants (k_5′), according to the RRKM theory of unimolecular reactions, yielded a high-pressure limiting activation energy E^∞_5′= 83.1 kJ mol^–1.

Thus the present data are consistent with a high-pressure limiting Arrhenius expression: log(k^∞_5′/s^–1)=(13.34±0.8)–[(10 000±1000)/2.303T] where the error limits are estimates of the accuracies of the Arrhenius parameters.

The results are discussed in relation to existing data on the CF₃CO radical and are compared with kinetic data on the decomposition reactions of the CH₃CO and C₂H₅CO radicals.